As compared with widely used amorphous silicon (a-Si), amorphous (non-crystalline) oxide semiconductors have high carrier mobility (also called as field-effect mobility, which may hereinafter be referred to simply as “mobility”), high optical band gaps, and film formability at low temperatures, and therefore, have highly been expected to be applied for next generation displays, which are required to have large sizes, high resolution, and high-speed drives; resin substrates having low heat resistance; and others.
Examples of the oxide semiconductors may include In-containing amorphous oxide semiconductors (e.g., In—Ga—Zn—O, In—Zn—O); however, since these oxide semiconductors contain In which is a rare metal, an increase in material cost is a serious concern in mass production process. Thus, as oxide semiconductors which contain no In, which can be produced at low material cost, and which are suitable for mass production, Zn—Sn—O (which may be represented by ZTO) oxide semiconductors made amorphous by addition of Sn to Zn have been proposed (e.g., Patent Document 1).
When an oxide semiconductor is used as a semiconductor layer of a thin-film transistor, the oxide semiconductor is required to have a high carrier concentration (mobility) and excellent TFT switching characteristics (transistor characteristics or TFT characteristics). More specifically, the oxide semiconductor is required to have, for example, (1) a high ON current (i.e., the maximum drain current when a positive voltage is applied to both a gate electrode and a drain electrode); (2) a low OFF current (i.e., a drain current when a negative voltage is applied to the gate electrode and a positive voltage is applied to the drain voltage, respectively); (3) a low S value (Subthreshold Swing, i.e., a gate voltage needed for a one-digit increase of the drain current); (4) a stable threshold value (i.e., a voltage at which the drain current starts to flow when a positive voltage is applied to the drain voltage and either a positive voltage or a negative voltage is applied to the gate voltage, which voltage may also be called as a threshold voltage) showing no change with time (which means that the threshold voltage is even in the substrate surface); and (5) a high mobility.
A particularly important one as showing TFT performance in the requirements described above is (5) mobility; and higher mobility provides higher TFT switching speed, so that higher performance transistors can be obtained. In recent years, display devices such as liquid crystal devices have rapidly proceeded to have large screens, high definition, high-speed drives, and others, with which semiconductor materials having high mobility have been eagerly desired. In this regard, mobility μ(cm2/Vs) is defined by the expression: V=μE using the speed V (cm/s) of carriers such as electrons and holes and the electric field E (V). Therefore, an increase in mobility needs a reduction in the content of defects in the semiconductor film to inhibit carrier scattering and lengthen carrier mean free path.
In this regard, carrier density can be adjusted by controlling the content of oxygen defects contained in the oxide semiconductor film. As the method of controlling the content of oxygen defects, there can be mentioned, for example, a method of adjusting the oxygen partial pressure in the process gas to be used when an oxide semiconductor film is formed by sputtering and a method of carrying out heat treatment in an oxygen or air atmosphere after the formation of an oxide semiconductor layer.
On the other hand, when an oxide semiconductor is applied to display devices and others, the securement of stable TFT characteristics needs the formation of a passivation layer on the surface of the oxide semiconductor. In general, insulator oxides such as SiOx, SiNx, SiON, and AlOx can be used as a passivation layer. For the formation of a passivation layer as described above, plasma CVD, sputtering, and other methods have widely been used. For example, the formation of a SiOx passivation layer by plasma CVD method is carried out, for example, by a method of depositing SiOx on the oxide semiconductor film, which SiOx is formed by the reaction of a mixed gas of SiH4 and N2O in the high-frequency plasma with an industrial frequency of 13.56 MHz.
However, if a passivation layer is formed by plasma CVD, sputtering, or other methods, radicals and molecules made to have high speed by plasma or others will collide to the surface of an oxide semiconductor; therefore, some defects may be formed in the surface of the oxide semiconductor. More specifically, the collision of gas components to be used in the formation of a passivation layer by plasma CVD or sputtering method, molecules accelerated to high speed by sputtering, and others, to the surface of the oxide semiconductor, thereby causing a phenomenon of desorption of oxygen contained in the oxide semiconductor (i.e., oxygen defects). These oxygen defects seriously affect TFT characteristics, such as an excessive carrier increase in the oxide semiconductor film, by which the oxide semiconductor film becomes conductive, thereby making it impossible to obtain stable switching characteristics, and by which a threshold voltage largely shifts to the negative side.
The explanation of an excessive carrier increase caused by oxygen defects was described above, but the same problem will occur in the case of hydrogen. For example, the diffusion of hydrogen, which is contained in a passivation layer such as made of SiOx or SiNx, into an oxide semiconductor film causes an excessive carrier increase, which adversely affects TFT characteristics.
To reduce such a deterioration in TFT characteristics due to oxygen and hydrogen, for example, non-patent literature document 1 proposes a method of excessively oxidizing an oxide semiconductor surface in advance by irradiating the oxide semiconductor surface with N2O plasma just before the formation of a passivation layer. However, this method is quite difficult in tuning because it is difficult to adjust conditions (e.g., input electric power, time, substrate temperature) for the irradiation of N2O plasma and because the irradiation conditions described above need to be adjusted according to the formation conditions and quality of a passivation layer and further according to the quality of an oxide semiconductor. In addition, a method as described above has a narrow process margin, possibly resulting in a decrease in yield, such as a variation in the substrate surface when TFT is produced with a large substrate and a change in TFT characteristics for each batch. Further, a method as described above may also cause other problems such as a decrease in productivity and an increase in production cost, for example, a need for the addition of a chamber for N2O plasma treatment before the formation of a passivation layer.